Method for direct voltage saturation calculation and prevention of inverter voltage saturation

ABSTRACT

A voltage saturation prevention algorithm used as at least part of a method of controlling an electric vehicle, wherein the electric vehicle comprises an electric motor, a controller, and an inverter. The controller receives a control signal with an instruction to operate the electric motor, then sends a switching signal corresponding to the control signal to the inverter, wherein the inverter provides a plurality of output signals for operation of the electric motor. The method includes determining the expected amplitude of the plurality of output signals based on the instruction to operate the electric motor, calculating the amount of modification of the plurality of output signals required to prevent the expected amplitude from reaching a saturation value, and modifying, based on the calculation, the instruction to operate the electric motor to prevent the expected amplitude from reaching the saturation value. The method is implemented in software, without any additional hardware.

CROSS REFERENCE TO PRIOR APPLICATIONS

This U.S. patent application is a continuation of, and claims priorityunder 35 U.S.C. § 120 from, U.S. patent application Ser. No. 16/560,657,filed on Sep. 4, 2019, which claims priority under 35 U.S.C. § 119(e) toU.S. Provisional Patent Application No. 62/833,733, filed on Apr. 14,2019. The disclosures of these prior applications is considered part ofthe disclosure of this application and are hereby incorporated byreference in their entireties.

TECHNICAL FIELD

The present disclosure relates to a voltage saturation preventionalgorithm.

BACKGROUND

Electric motors are a main part of the powertrain of electric vehicles.These motors are controlled by power inverters that have limited inputand output voltage. One of the main problems of inverter operation athigh speeds is voltage saturation, a condition where the requestedvoltage from the inverter is beyond the inverter's possible deliverablevoltage. In this condition, the drive system becomes oscillatory andeven unstable in severe transient cases.

Prior arts have utilized different approaches to overcome the problem ofvoltage saturation in inverters. In one approach, voltage derating(setting the allowable voltage maximum to a point below the absolutevoltage maximum) is applied on the calibration tables of motor controlsoftware. However, this approach by definition prevents the inverterfrom providing its rated maximum capability. In other cases, onlinevoltage saturation algorithms are employed. However, the previouslyproposed methods have a complex structure and are computationallyexpensive.

SUMMARY

To overcome these shortcomings, this document presents a novel voltagesaturation prevention method for permanent magnet (PM) motor drives. Inone embodiment, the method directly calculates the amount ofmodification in current commands of the inverter to provide maximumvoltage capability of the inverter and simultaneously prevents theinverter from voltage saturation. In one embodiment, the method is basedon current angle modification. In one embodiment, the method alsoaddresses possible errors in calibration tables caused by look-up tableinterpolations. In one embodiment, to implement this method, amathematical analysis may be performed first to study the behavior ofvoltage components of the inverter at high-speed operation.

Disclosed herein is a voltage saturation prevention algorithm used as atleast part of a method of controlling an electric vehicle, wherein theelectric vehicle comprises an electric motor, a controller, and aninverter. In one embodiment, the controller receives a control signalwith an instruction to operate the electric motor. In one embodiment,the inverter receives a switching signal corresponding to the controlsignal from the controller, the inverter providing a plurality of outputsignals for operation of the electric motor.

In one embodiment, the method includes determining the expectedamplitude of the plurality of control signals based on the instructionto operate the electric motor. In one embodiment, the method includescalculating the amount of modification of the plurality of controlsignals required to prevent the expected amplitude of the output signalsfrom reaching a saturation value. In one embodiment, the method includesmodifying, based on the calculation, the instruction to operate theelectric motor to prevent the expected amplitude from reaching thesaturation value. In one embodiment, the method is implemented insoftware.

In another embodiment, the instruction is an instruction for theinverter to operate at a specific command current value. In anotherembodiment, the specific command current value has a plurality ofcurrent components.

In another embodiment, the plurality of output signals is a plurality ofvoltage components.

In another embodiment, calculating is based on current anglemodification. In another embodiment, a modification in current angleresults in reduction of voltage amplitude. In another disclosedembodiment, the amount of modification in current angle (andconsequently in current components) is calculated from a detailedmathematical analysis.

In another embodiment, the method automatically corrects the errors in acalibration table of the electric motor. By using the proposed voltagesaturation prevention algorithm, the calibration tables of motor controlsoftware may be defined based on maximum capability of the inverter(without considering conservative deratings), and the proposed algorithmmay protect the inverter from oscillatory operation beyond its linearrange.

According to one embodiment, the proposed algorithm has a simplestructure and directly calculates the amount of modification in currentcomponents from the amount of voltage saturation. In another embodiment,the proposed approach automatically corrects the errors in calibrationtables of the motor while allowing the motor to operate up to itsmaximum capability. In another embodiment, the simple structure of theproposed algorithm eases its implementation and computational load onthe inverter-motor system's microprocessor.

In another disclosed embodiment, the voltage saturation preventionalgorithm is based on observations from a mathematical analysis ofvoltage components in an electric motor drive system. In anotherembodiment, based on the proposed mathematical analysis, when the q-axisvoltage component is positive, an increase in current angle reduces thevoltage amplitude and may overcome the problem of voltage saturation. Inaddition, in one embodiment, when the q-axis voltage component isnegative, for the active high-voltage operating range of the inverter,an increase in absolute current angle still may reduce the impact oroccurrence of the voltage saturation problem. In another disclosedembodiment, the amount of modification in current angle (andconsequently in current components) is calculated from a detailedmathematical analysis. In one embodiment, by using the proposed voltagesaturation prevention algorithm, the calibration tables of motor controlsoftware may be defined based on maximum capability of the inverter(without considering conservative deratings), and the proposed algorithmmay protect the inverter from oscillatory operation beyond its linearrange.

Other aspects, features, and techniques will be apparent to one skilledin the relevant art in view of the following detailed description of theembodiments.

DESCRIPTION OF DRAWINGS

The features, objects, and advantages of the disclosed embodiments willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a perspective view depicting an exemplary embodiment of anelectric vehicle that may include an electric motor drive system.

FIG. 2 is a flow diagram depicting the structure of an exemplaryelectric motor drive system in electric vehicles.

FIG. 3A is a side view depicting an exemplary embodiment of an electricmotor for an electric vehicle. FIG. 3B is a side view depicting anexemplary current vector coordinate system for the electric motor ofFIG. 3A.

FIG. 4 is a signal block diagram depicting an exemplary embodiment of avoltage saturation prevention algorithm.

DETAILED DESCRIPTION

One aspect of the disclosure is directed to a voltage saturationprevention algorithm used as at least part of a method of controlling anelectric vehicle.

References throughout this document to “one embodiment,” “certainembodiments,” “an embodiment,” or similar term mean that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof such phrases in various places throughout this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner on one or more embodiments without limitation. Forexample, two or more of the innovative algorithms described herein maybe combined in a single implementation, but the application is notlimited to the specific exemplary combinations of voltage saturationprevention algorithms that are described herein.

As used herein, the terms “a” or “an” shall mean one or more than one.The term “plurality” shall mean two or more than two. The term “another”is defined as a second or more. The terms “including” and/or “having”are open ended (e.g., comprising). The term “or” as used herein is to beinterpreted as inclusive or meaning any one or any combination.Therefore, “A, B or C” means “any of the following: A; B; C; A and B; Aand C; B and C; A, B and C”. An exception to this definition will occuronly when a combination of elements, functions, steps or acts are insome way inherently mutually exclusive.

A detailed description of various embodiments is provided; however, itis to be understood that the disclosed embodiments are merely exemplaryand may be embodied in various and alternative forms. The figures arenot necessarily to scale; some features may be exaggerated or minimizedto show details of particular components. Therefore, specific structuraland functional details disclosed herein are not to be interpreted aslimiting, but merely as a representative basis for teaching one skilledin the art to variously employ the disclosed embodiments.

FIG. 1 is a perspective view depicting an exemplary embodiment of anelectric vehicle 100 that may include an electric motor drive system200. The electric vehicle 100 shown in FIG. 1 is exemplary. The electricmotor drive system 200 may be installed in any vehicle with use for anelectric motor drive system, including but not limited to hybridvehicles.

FIG. 2 is a flow diagram depicting the structure of an exemplaryelectric motor drive system 200 in electric vehicles. The algorithmdepicted in FIG. 4 may be installed as part of the inverter controlboard 210, which in one embodiment is designed to control the inverter220 at its commanded value with a good dynamic and steady-stateresponse. In one embodiment, the inverter 220 feeds power to theelectric motor 230, which in turn applies torque to a plurality ofvehicle wheels 240.

In one embodiment, the inverter control board 210 receives a controlsignal with an instruction to operate the electric motor 230. In oneembodiment, the control signal comprises a torque command. In oneembodiment, the torque command may be an instruction for the electricmotor 230 to operate with a specific torque value in order to achieve adesired velocity for the electric vehicle 100. The torque command may bedetermined based on input from systems including but not limited to anacceleration pedal of the electric vehicle 100, a brake pedal of theelectric vehicle 100, and a cruise control system of the electricvehicle 100. In one embodiment, the inverter control board may receivefurther feedback regarding the operating conditions of the electricmotor drive system 200 or other systems by means of an analog input or aresolver input.

In one embodiment, the inverter control board 210 performs a series ofcalculations using the received instruction in order to produce aswitching signal for the inverter 220. The switching signal may bedesigned to use space vector pulse width modulation (SVPWM) to operatethe legs of the inverter 220 to produce a plurality of output signalscorresponding to the received instruction to operate the electric motor230. In one embodiment, the plurality of output signals is a set ofcurrent components. In one embodiment, the plurality of output signalsincludes a first output signal and a second output signal, wherein thefirst output signal is a current component that is related to the directaxis as a flux component, and wherein the second output signal is acurrent component that is related to the quadrature axis as a torquecomponent. In one embodiment, the plurality of output signals may bemapped to a dq frame of reference.

FIG. 3A is a side view depicting an exemplary embodiment of an electricmotor 230 for an electric vehicle 100. In one embodiment, the electricmotor 230 includes a rotor 231 and a stator with a plurality of statorcoils 232. In one embodiment, the rotor 231 may include a permanentmagnet. In one embodiment, the stator coils 232 may receive power fromthe inverter 220 to produce a magnetic field by means of SVPWM. In oneembodiment, the electric motor drive system 200 rotates the magneticfield of the stator in order to induce rotation in the rotor 231 andpropel the electric vehicle 100.

FIG. 3B is a side view depicting an exemplary current vector coordinatesystem for the electric motor 230 of FIG. 3A. In one embodiment, theelectric motor 230 has a rotation angle value θ. Rotation angle θ mayrepresent a current angle of the motor, wherein the current angle is theangle between the current vector i_(dq) and the d-axis. In oneembodiment, the current angle is defined as

$\theta = {a{\tan\left( \frac{i_{q}}{i_{d}} \right)}}$

FIG. 4 is a signal block diagram depicting an exemplary embodiment of avoltage saturation prevention algorithm 300. According to oneembodiment, the voltage saturation prevention algorithm 300 modifies thecommanded current components comprising the output signal only when thevoltage amplitude is saturated and exceeds a maximum allowable value.

In one embodiment, the modulation index m is defined as

$m = \frac{\sqrt[2]{V_{d}^{2} + V_{q}^{2}}}{V_{DC}}$

and modulation index differential m_(diff) may be expressed with thefollowing equation:

m _(diff) =m−m _(max)

where m_(max) is the maximum allowable modulation index value before theelectric motor drive system 200 is considered to be experiencing voltagesaturation. m_(max) may vary according to a number of factors, includingbut not limited to the hardware architecture of the inverter and the PWMstrategy.

In one embodiment, saturated voltage modifier E_(compensator) is theresult of feeding m_(diff) through a compensator. The compensator may bea proportional (P) compensator, proportional-integral (PI) compensator,or any other compensator type chosen by the designer. In one embodiment,the compensator uses windup or non-windup limiters to limitE_(compensator) to a minimum and maximum allowable value. In oneembodiment, the minimum allowable value of E_(compensator) is 0,representing a condition of no voltage saturation and normal electricmotor operation. In one embodiment, the maximum allowable value ofE_(compensator) is E_(max) (wherein E_(max) is an application-specificdesign parameter), representing a condition of maximum voltagesaturation adjustment.

In one embodiment, the modified quadrature current componentI_(q,cmd,modified) may be expressed with the following equation:

I _(q,cmd,modified) −I _(q,cmd) −I _(q,cmd) *E _(compensator)

wherein I_(q,cmd) is the original current component before factoring involtage saturation. The above equation may be reduced to the followingequation:

I _(q,cmd,modified) −I _(q,cmd)(1−E _(compensator))

In one embodiment, E_(compensator) is equal to zero andI_(q,cmd,modified) is equal to I_(q,cmd). In this condition, the voltagesaturation prevention algorithm 300 has determined that the electricmotor drive system 200 is not experiencing voltage saturation and nocurrent modification is necessary. In one embodiment, E_(compensator) isnot equal to zero and I_(q,cmd,modified) has a magnitude equal to afraction of the magnitude of I_(q,cmd). In this condition, the voltagesaturation prevention algorithm 300 has determined that the electricmotor drive system 200 is experiencing voltage saturation and that amodification of current value is necessary to prevent that condition.

In one embodiment, the modified direct current componentI_(d,cmd,modified) may be expressed with the following equation:

$I_{d,{cmd},{modified}} = {I_{d,{cmd}} - {I_{q,{cmd}}*\left( {I_{q,{cmd}}E_{compensator}} \right)\frac{1}{{❘I_{d}❘}_{\max}}}}$

wherein I_(d,cmd) is the original direct current command beforefactoring in voltage saturation and |I_(d)|max is the maximum potentialvalue of the direct current component that the electric motor drivesystem 200 may have. The above equation may be reduced to the followingequation:

$I_{d,{cmd},{modified}} = {I_{d,{cmd}} - \frac{I_{q,{cmd}}^{2}E_{compensator}}{{❘I_{d}❘}_{\max}}}$

In one embodiment, E_(compensator) is equal to zero andI_(d,cmd,modified) is equal to I_(d,cmd). In this condition, the voltagesaturation prevention algorithm 300 has determined that the electricmotor drive system 200 is not experiencing voltage saturation and nocurrent modification is necessary. In one embodiment, E_(compensator) isnot equal to zero and I_(d,cmd,modified) has a magnitude greater thanthe magnitude of I_(d,cmd). In this condition, the voltage saturationprevention algorithm 300 has determined that the electric motor drivesystem 200 is experiencing voltage saturation and that a modification ofcurrent value is necessary to prevent that condition.

In one embodiment, m is determined by the inverter control board 210 asa function of the current component values I_(d,cmd) and I_(q,cmd). Inone embodiment, the ability to determine m without using measurementsystems allows the voltage saturation prevention algorithm 300 tooperate without adding any additional hardware to the electric motordrive system 200.

While this disclosure makes reference to exemplary embodiments, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the scope of theclaimed embodiments.

What is claimed is:
 1. A computer-implemented method executed on dataprocessing hardware of a vehicle that causes the data processinghardware to perform operations comprising: receiving a control signalcomprising a specific command current value to operate the electricmotor, the specific command current value comprising: a commandedquadratic current component value I_(q,cmd); and a commanded directcurrent component value I_(d.cmd), wherein a direct current componentfor the electric motor comprises a maximum potential value |Id|max;calculating a saturated voltage modifier value E_(compensator) forpreventing an expected amplitude of a plurality of output signals for aninverter from reaching a saturation value; and modifying the specificcommand current value to operate the vehicle by: determining a modifiedcommanded quadratic current component value I_(q,cmd,modified) bymultiplying the commanded quadratic current component value I_(q,cmd) bythe saturated voltage modifier value E_(compensator); and determining amodified direct current component value I_(d,cmd,modified) bycalculating:${Id},{{cmd} - {\frac{{Iq},{{cmd}^{2} \times {Ecompensator}}}{{❘{Id}❘}\max}.}}$2. The method of claim 1, wherein the vehicle comprises an electricvehicle.
 3. The method of claim 1, wherein the plurality of outputsignals is a plurality of voltage components.
 4. The method of claim 1,wherein calculating the saturation voltage modifier value is based on acurrent angle modification.
 5. The method of claim 4, wherein amodification in the current angle results in a reduction of amplitude ofthe plurality of output signals
 6. The method of claim 1, wherein theoperations further comprise determining a modulation index based on thespecific command current value.
 7. The method of claim 6, wherein themodulation index is determined as a function of the specific commandcurrent value without using any measurement systems
 8. The method ofclaim 6, wherein calculating the saturation voltage modifier value isbased on the modulation index.
 9. The method of claim 1, wherein theoperations further comprise automatically correcting errors in acalibration table of the electric motor based on the specific commandcurrent value.
 10. The method of claim 1, wherein the calculatedsaturated voltage modifier value is limited to a maximum allowable valuebased on an application-specific design parameter.
 11. A vehiclecomprising: an electric motor; an inverter providing a plurality ofoutput signals for operation of the electric motor; and a controller incommunication with the inverter and performing operations comprising:receiving a control signal comprising a specific command current valueto operate the electric motor, the specific command current valuecomprising: a commanded quadratic current component value I_(q,cmd); anda commanded direct current component value I_(d.cmad), wherein a directcurrent component for the electric motor comprises a maximum potentialvalue |Id|max; calculating a saturated voltage modifier valueE_(compensator) for preventing an expected amplitude of a plurality ofoutput signals for an inverter from reaching a saturation value; andmodifying the specific command current value to operate the vehicle by:determining a modified commanded quadratic current component valueI_(q,cmd,modified) by multiplying the commanded quadratic currentcomponent value I_(q,cmd) by the saturated voltage modifier valueE_(compensator); and determining a modified direct current componentvalue I_(d,cmd,modified) by calculating:${Id},{{cmd} - {\frac{{Iq},{{cmd}^{2} \times {Ecompensator}}}{{❘{Id}❘}\max}.}}$12. The vehicle of claim 11, wherein the vehicle comprises an electricvehicle.
 13. The vehicle of claim 11, wherein the plurality of outputsignals is a plurality of voltage components.
 14. The vehicle of claim11, wherein calculating the saturation voltage modifier value is basedon a current angle modification.
 15. The vehicle of claim 14, wherein amodification in the current angle results in a reduction of amplitude ofthe plurality of output signals
 16. The vehicle of claim 11, wherein theoperations further comprise determining a modulation index based on thespecific command current value.
 17. The vehicle of claim 16, wherein themodulation index is determined as a function of the specific commandcurrent value without using any measurement systems
 18. The vehicle ofclaim 16, wherein calculating the saturation voltage modifier value isbased on the modulation index.
 19. The vehicle of claim 11, wherein theoperations further comprise automatically correcting errors in acalibration table of the electric motor based on the specific commandcurrent value.
 20. The vehicle of claim 11, wherein the calculatedsaturated voltage modifier value is limited to a maximum allowable valuebased on an application-specific design parameter.